Motor vehicles, such as automobiles and smaller trucks, are provided with a parking brake control which is used to set the brakes of the vehicle at certain times, such as when the vehicle is parked. These parking or emergency brake controls generally include a foot pedal which pivots a lever that pulls on a cable for setting the brakes of the vehicle. When the pedal has been depressed, an element on the brake control locks the brakes in the set or activated condition. A variety of mechanisms have been employed for this purpose; however, one of the most common mechanism is a pivoted pawl which engages the arcuately arranged teeth of a gear sector carried by the pedal operated lever. This pawl engages the teeth of the gear sector to hold the brakes in the engaged condition. In this manner, the brakes are set upon depression of the foot pedal by the interaction of the pivoted pawl and gear sector. The parking or emergency brake is released by disengaging the pawl and sector which allows the force being applied by the brake to pull the parking brake control into the released or deactivated condition. The release of the pawl has been accomplished by a variety of mechanisms. The least complex arrangement is a manually operated handle which pulls the pawl from engagement with the gear sector. In this manner, the parking or emergency brake is released by the person operating the vehicle. Although this type of control for the parking and emergency brakes seems to be quite simple, there was one disadvantage realized early in its development. The forces between the gear sector and locking pawl are often quite high; therefore, the manual force necessary to release the parking or emergency brake by disengaging the pawl was often beyond acceptable or convenient levels. For that reason, a substantial amount of work has been devoted to arrangements for disengaging the pawl and gear sector. Such a mechanism is illustrated in U.S. Pat. No. 3,135,133 wherein the gear sector itself is shifted to release the brake locking action. This patent, which is incorporated by reference herein, is an example of one arrangement for manually releasing the parking brakes of a motor vehicle. It illustrates the complexity of rectifying the force induced difficulties experienced by the commonly used pawl and gear sector.
The automotive industry, in keeping with an effort to provide power assist to all manual control functions, demanded a power unit for releasing the parking or emergency brakes of an automobile. The power unit developed was a vacuum operator such as shown generally in U.S. Pat. No. 3,315,538. One difficulty experienced by such vacuum operated release mechanisms was that, in order to create the force necessary to release a pawl from a gear sector, the vacuum unit or motor had to have a substantial size because the only vacuum available was the vacuum created by the manifold system. These large vacuum units or motors were not acceptable. They required too much space and were costly to produce. Thus, there was a trend to smaller vacuum units or motors, as shown in U.S. Pat No. 3,315,538; however, these units required a modified brake locking mechanism. Such a small vacuum unit operated by the vacuum of an internal combustion engine could not operate the normal pawl release. One modified release mechanism includes a pinion gear that rotates with the gear sector. The pinion is locked into position by a surrounding spring. Such a friction locking device can be operated at a lesser force than required in the past for disengaging a pawl from a gear sector.
After having developed a relatively small vacuum motor and a low force friction release device, these mechanisms were used for automatically releasing the brake when the transmission is shifted into a drive condition. This automatic release concept is also disclosed in the aforementioned U.S. Pat. No. 3,315,538. The relatively small vacuum motor was controlled by a valve operated from the transmission control to release the brakes when the transmission control was shifted to the drive or reverse position. Such automatic release mechanisms were quite successful and were used by automobile manufacturers. However, some difficulties were experienced when the brakes were to be released while there was no vacuum. This condition occurred when the engine was not operating. For instance, if an automobile with an automatic brake release was stalled with the brakes applied, the automobile could not be pushed or towed in a conventional fashion. To correct this problem, the automatic brake release mechanisms using fluid motors usually employed manual release levers. These levers were often not easily accessible and were not known generally to persons other than the usual driver.
Even with the problems mentioned above, the automatically released parking brakes using vacuum motors and a friction locking mechanism became somewhat accepted in the trade. Efforts were made to improve the vacuum operated unit or motor for this purpose. A patent relating to such an improvement is U.S. Pat. No. 3,333,512, which patent is also incorporated by reference herein.
To overcome the difficulties experienced with vacuum operated automatic brake release mechanisms, efforts were devoted to convert such release systems to an electrical system wherein a solenoid could be used to disengage the parking or emergency brake. This concept is shown in U.S. Pat. No. 3,358,797. Again, only a relatively small pulling force was available. This patent illustrates the concept of a parking brake being released by a cam movable in unison with a standard transmission control. When the transmission control is in the drive or reverse position, the brakes are automatically released.